This is a proposal for a Mentored Research Scientist Development Award. It is the investigator's long term career goal to become an independent scientist who designs and implements peer-based HIV-prevention interventions for multicultural adolescents at risk for HIV, especially homeless youth. HIV prevalence is alarmingly high among homeless youth in the United States, with levels as high as 10% in some urban settings. These youth, who in Los Angeles County and most other urban settings in the United States are disproportionately comprised of racial/ethnic minority youth, are at an alarmingly high risk for contracting HIV/AIDS. The problems faced by these youth are varied and complex. Homeless youth require a variety of intervention approaches to meet their unique needs. The proposal focuses on developing a social network-based HIV prevention intervention for homeless youth. The project is divided into three sequential phases, where each training goal is matched with a phase of research where the investigator will have an opportunity to apply his new skills. The training goals are: 1. To gain expertise in adolescent developmental psychology, especially issues of peer influence. 2. To gain experience in qualitative methods, especially in-depth interviewing techniques. 3. To gain expertise in intervention design, particularly opinion leader/peer-based interventions. The goal of the research project is to learn how the social networks of homeless youth can positively shape sexual and substance use behaviors and to pilot an intervention to enhance those processes. The following specific aims guide the three phases of the research proposal: 1. To assess the prevalence, social status, and social roles of pro-social peers in the social networks of homeless youth (Phase 1: Ego-Centric Networks). 2. To model social network structures and the social influence process among homeless youth (Phase 2: In-Depth Interviews). 3. To develop an intervention manual and pilot test a peer-based HIV prevention intervention for homeless youth which mobilizes and augments the positive influence of pro-social peers (Phase 3: Manual Development and Pilot Testing).